Many electromagnetic machines in general, and permanent magnet electric motors in particular, exhibit torque irregularities as the rotor rotates with respect to the stator. Such irregularities produce non-uniform torque output and, thus, non-uniform rotation of the rotor. These torque irregularities may be caused by the physical construction of a given machine. They can result from, for example, a bearing that sticks in a given rotor position or the fact that, because of the electromagnetic characteristics of the machine, the rotor tends to prefer certain angular positions with respect to the stator. Torque irregularities resulting from the electromagnetic characteristics of an electromagnetic machine are commonly known as "cogging" irregularities and the resultant non-uniform rotation of the rotor or non-uniform torque output is known as "cogging."
In permanent magnet machines, cogging most often results from the physical construction of the machine. Specifically, the utilization of rotors having discrete north and south outer poles results in a circumferential distribution of magnetic flux about the rotor circumference that is not smooth, but choppy. Additionally, the stators commonly used with such machines are formed in such a way that the magnetic fluxes generated by the stator windings provide a flux distribution about the stator circumference that is not smooth. The combination of such rotors and stators, and the accompanying non-smooth flux distributions, produces undesired cogging irregularities. Rotor output non-uniformities may also be produced by back emf harmonics produced in certain machines.
For many motor applications the slight non-uniformity in the rotation of the rotor caused by torque irregularities is of little or no consequence. For example, in large motors driving large loads, slight variations in the output torque will not significantly affect the rotor speed and any slight variations in rotor speed will not significantly affect the system being driven by the machine. In other applications, where the rotation of the rotor or the torque output of the motor must be precisely controlled or uniform, such non-uniformity is not acceptable. For example, in servomotors used in electric power steering systems and in disk drives, the rotational output of the rotor or the torque output of the motor must be smooth and without significant variation. In many such applications the maximum acceptable peak-to-peak torque ripple or rotational speed ripple as a percentage of full load mean is on the order of only 1% to 2%. Because such machines typically have small, relatively low mass rotors, torque irregularities of a small magnitude can adversely impact the output of the machine.
Prior art approaches to reducing the undesirable consequences of torque irregularities in electromagnetic machines have focused on relatively complex rotor or stator constructions designed to eliminate the physical characteristics of the machines that would otherwise give rise to the irregularities. While the prior art machine construction approaches can result in reduction of torque irregularities, the approaches require the design and construction of complex rotor and stator components, such complex components are typically difficult to design, difficult to manufacturer, and much more costly to produce than are conventionally constructed components. Moreover, many of the physical changes required by such prior art solutions result in a significant reduction in the efficiency or other performance parameters of the resulting machines over that expected of comparable conventional machines. Thus, many of the prior art attempts to reduce torque irregularities do so at the cost of machine performance.
It is an object of the present invention to provide an improved method and apparatus for reducing the negative consequences of torque irregularities that do not suffer from the described and other limitations associated with the prior art.